1. Field of Invention
The present invention relates to an ignition coil for an internal combustion engine for use in a vehicle or the like, and to a method of manufacturing the ignition coil for an internal combustion engine.
2. Description of Related Art
It is known to provide an independent ignition type-ignition coil for each cylinder of an internal combustion engine. One ignition coil of this kind is shown in FIGS. 20 and 21 and is of the so-called upward-set type in which a casing 116 is installed on an opening of a plug hole Ha formed in a cylinder head H of an internal combustion engine.
The casing 116 accommodates a coil 114 of a concentric type including a primary coil section 111 with an enamel wire wound on the periphery of a magnetic core 110 and a secondary coil section 112 with an enamel wire wound on the periphery of the primary coil section 111, with the magnetic core 110 being kept horizontal. A high secondary voltage generated in the secondary coil section 112 of the coil 114 is applied to an ignition plug P positioned at the bottom of the plug hole Ha through a connection member 118 accommodated inside the plug hole Ha.
In recent years, there has been a growing demand for reduction of the height of the part of the ignition coil projecting from the cylinder head H. This is because when the height of this projecting part is large, as shown in FIGS. 20 and 21, it may interfere with suction and exhaust component parts accommodated inside the engine room.
To overcome this problem, as shown in FIG. 22, a so-called in-hole type ignition coil has been proposed. This has a concentric-type coil 103 including a primary coil section 101 with an enamel wire wound on the periphery of a rod-shaped magnetic core 100 and a secondary coil section 102 with an enamel wire wound on the periphery of the primary coil section 101 (or the secondary coil section 102 is formed on the periphery of the magnetic core 100, and the primary coil section 101 is formed on the periphery of the secondary coil section 102). A plug hole Ha accommodates the coil 103. A high secondary voltage generated in the secondary coil section 102 is applied to an ignition plug P positioned at the bottom of the plug hole Ha through a connection member 108 accommodated inside the plug hole Ha.
In this type of ignition coil, because the coil 103 is accommodated inside the plug hole Ha, it is possible to reduce the height of a part 112 projecting from the plug hole Ha.
However, generally, the inner diameter of the plug hole Ha is as small as 23-24 mm. Thus, in the ignition coil shown in FIG. 22, there are restrictions on the thickness of the enamel wire forming the primary coil section 101 and the secondary coil section 102, the number of turns of the enamel wire, and the layout of the magnetic core 100. Thus, it is impossible for the ignition coil to generate a sufficiently great secondary energy.
In particular, in recent years, the direct fuel injection type of internal combustion engine has been rapidly widely adopted. In this type of engine, the ignition coil is required to generate large secondary energy in order to ignite a gas mixture in the cylinder. The ignition coil shown in FIG. 22 is incapable of satisfying such a demand to a sufficient extent. Another problem is that, because the coil 103 is accommodated in the narrow and closed plug hole Ha, the ignition coil is inferior in heat-radiating performance.
In order to overcome the problem the present inventors have devised, but not made public, an ignition coil, such as is shown in FIG. 23, including a coil 124 having a primary coil section 121 and a secondary coil section 122 formed on the periphery of the primary coil section 121. The coil 124 is of concentric type and laterally flat. The coil 124 is installed on a plug hole Ha (e.g., see FIGS. 20 and 22), with the lateral (flat) direction being horizontal. However, it has been revealed that in order to secure a secondary energy having the required magnitude, it is necessary to considerably increase the number of turns of an enamel wire forming the primary and secondary coil sections 121 and 122. When the number of turns of the enamel wire is increased, the ignition coil becomes large radially. Consequently, the ignition coils interfere with each other when assembled adjacent to each other on the engine head.
In a known kind of ignition coil for an internal combustion engine, an enamel wire of circular cross-section is used as a coil winding to be wound on a primary coil winding seat and a secondary coil winding seat.
FIGS. 31 and 32 show conventional methods of winding such an enamel wire of circular cross-section on the primary and secondary coil winding seats.
FIG. 31 shows a cross-section of a coil in which a coil winding 410 such as an enamel wire having a diameter of 0.5 mm is wound 80 times between a pair of flange portions 402 of a primary coil winding seat 404. In this method, the wire of the coil winding 410 is wound between the two flange portions 402. The wire is wound such that, as viewed in cross-section, the winding displays columns of four circles (each circle being a cross-section of the wire), each circle of a column being at the same level as a respective circle of an adjacent column.
FIG. 32 shows a cross-section of a coil in which a coil winding 410 such as an enamel wire having a diameter of 0.5 mm is wound 81 times between a pair of the flange portions 402 of the primary coil winding seat 404. In this method, the coil winding 410 is wound so that in cross-section the winding displays alternating columns of three and four circles, each circle of a column being displaced in the direction between the flanges 402 from a respective circle in a neighbouring column by a distance equal to the radius of the coil winding 410.
In the above-described conventional ignition coils for an internal combustion engine, the coil winding 410 is circular in cross-section. Thus, even though the coil winding 410 is packed tightly between both flange portions 402, gaps are formed between the adjacent rounds of the coil winding 410. Consequently, the size of the ignition coil for an internal combustion engine is increased according to the size of the gaps.
Also, each gap is filled with air. The heat conductivity of air is lower than that of the coil winding 410 of enamel wire. Thus, heat generated at the primary coil during the use of the ignition coil is not radiated efficiently and promptly.